In recent years, along with widespread use of portable personal computers and portable phones, it has been highly demanded to develop an electricity storage device, such as a lithium ion secondary battery, having a higher capacity and a reduced size. The electricity storage device having a higher capacity facilitates reduction in size of a battery, and hence the development of the electricity storage device having a higher capacity is urgently needed.
As a negative electrode active material for an electricity storage device, such as a lithium ion secondary battery or a sodium ion secondary battery, carbon materials, such as a graphite carbon material and hard carbon, have been generally used.
Further, as a negative electrode active material capable of storing and releasing lithium ions or sodium ions, layered sodium titanium oxide Na2Ti3O7 has been proposed (see Patent Literature 1).